The glass tinting industry desires a class of solar energy control coatings or films with a visual light transmission (VLT) less than 50%, and preferably less than 30%. At the same time, the industry desires these coatings or films to have a visual light reflection (VLR) less than 15%, and preferably equal to or less than 10%.
With the metallized plastic films conventionally employed for solar energy control in the window glass industry, the visual light transmittance or VLT can be decreased by increasing the thickness of the metal layer on the film, but this results in an increase in the visual light reflectance or VLR. For example, typical metal coated solar films with a VLT of 25% have a VLR of 30 to 35% and more. Thus, the VLT and the VLR become competing interests without a middle of the road compromise acceptable to the industry. In general, at the present time, the VLT is set at an acceptable level, and the VLR remains higher than desired.
An alternate approach to the demands of the industry has been to utilize dyed plastic films or sheets, either alone or as a substrate for a metal film or layer. However, dyed films have very poor solar performance and the color fades with aging. Consequently, the dyed films have not provided a satisfactory solution to the demands of the industry.
Another attempt to reduce the VLR of low VLT metallized films has been to apply coatings of titanium oxide or indium tin oxide adjacent the film or layer of metal to control reflection within a narrow spectral band. According to classical optics, sandwiching of the metal film between layers of a material of high refractive index can boost visual transmission, that is, so-called induced transmission, and reduce reflection. In common practice, this requires 70 to 100 nanometer thick layers of titanium oxide or indium tin oxide, which are very slow to produce and difficult to control. As a result, this approach has proven too expensive to be practical, and even so, provides at best only a partial solution to the VLT/VLR dichotomy.
U.S. Pat. No. 4,799,745 (Reexamination Certificate B1 4799745) discloses an infrared reflecting film employing Fabry-Perot interference filters comprised of two or more transparent layers of metal, such as silver, gold, platinum, palladium, aluminum, copper, nickel and alloys thereof, separated by directly contiguous intervening dielectric spacer layers, which may suitably be the oxides of indium, tin, titanium, silicon, chromium and bismuth. Related U.S. Pat. No. 5,071,206, which issued on a continuation-in-part of patent 4,799,745, discloses a color corrected infrared reflecting film comprised of a substrate bearing seven directly contiguous alternating layers of dielectric and silver. While these films have low visual light reflectance, they require five to seven layers of material sputter deposited onto one another, which is expensive and not easy to accomplish. With decreasing visual light transmission, the task becomes more difficult.
Patent Cooperation Treaty international publication WO 94/04356 teaches that the reflectivity of a carbon based polymer sheet may be reduced by sputter-depositing onto the sheet a discontinuous dendritic layer of inorganic material having an index of refraction greater than that of the polymer. The inorganic material may be an oxide, nitride or oxynitride of a primary metal selected from tantalum, niobium, titanium, hafnium, tungsten and zirconium. The primary coating may be supplemented by an overcoat layer comprising an oxide, nitride or oxynitride of a secondary metal selected from indium, tin and zinc. When used as internal plastic surfaces in multi-pane window units, the coatings increase light transmission through the polymer film with minimal coloration and haze. Thus, while reflection is reduced, transmission is not.
Consequently, there remains in the industry a strong demand for development of an inexpensive class of coatings and/or coated films that will have both a low VLT and a low VLR.